1. Field of the Invention
The present invention relates to architectures and designs of Global Positioning System (GPS) receiving systems. In particular, the invention relates to the design of multiple-beam antennas using digital beam forming techniques to enable dynamic tracking of all GPS satellites within the field of view of the aperture.
2. Description of Related Art
It is well known in the art to use the NAVSTAR GPS satellite constellation to determine receiver location and obtain navigation information. The basic NAVSTAR constellation comprises twenty-four satellites orbiting in six orbital planes. Each orbital plane is inclined with respect to the equator by fifty-five degrees, and they are separated by sixty degrees right ascension. In each of the six orbital planes, four satellites are evenly spaced along a nearly circular orbit. This arrangement assures that between four and twelve satellites are visible above an observer's horizon at any given time. However, the satellites will be observed near the observer's horizon the majority of the time. FIG. 1 illustrates the relative time that a GPS satellite passing through the local zenith will be visible as a function of elevation angle. A user has about 40% more time to observe the GPS satellite traveling over an elevation angle near the local horizon than that near the local zenith, and for satellites not reaching local zenith, the time spent near the local horizon is even greater. For airborne receiving platforms in particular, the majority of GPS signals arrive at low elevation angles. However, typical GPS receiving systems employ largely isotropic, low-gain antennas in order to simultaneously view large regions of the sky. Besides wasting much of the antenna gain in directions with low payoff, such isotropic receivers also increase the system's susceptibility to interference signals, especially during aircraft final approach and landing. Therefore, it would be desirable to design antennas for receiving GPS signals that exhibit better directional gain and that are better able to discriminate toward low elevation angles.
The low broadcast power of GPS signals makes them susceptible to interference. The interference/signal (I/S) power ratio is a function of the distance and transmission power of the interfering source. Because the current generation of GPS terminals relies on using the relatively susceptible coarse acquisition (C/A) code for signal acquisition, receivers are readily disabled by interfering sources. Even low-power interfering sources can have a drastic effect on GPS receivers at a significant distance from the interfering source. Conventional low-gain antennas cannot discriminate between GPS signal sources and interfering sources. Thus, it would be desirable to provide a receiving antenna system that could dynamically provide gain in the direction of the desired GPS sources while suppressing interfering sources by suppressing antenna gain in the directions of the interfering sources.